There has been rapid progress in performance and diversification of electronic devices in recent years. In pace with the progress, there has been an upward surge in number of devices equipped with optically transparent touch panels in front of display devices such as LCD's and the like. In any such device, a user visually catches and selects characters, marks, signs, and the like displayed in a display device through a touch panel, and switches individual functions by operating the optically transparent touch panel.
Referring to FIG. 2, a description is provided of a conventional optically transparent touch panel of such kind. FIG. 2 is a sectional view of a conventional optically transparent touch panel. In the figure, optically transparent upper substrate 11 is provided with optically transparent upper conductive layer 12 formed on a lower surface thereof. A pair of upper electrodes 17 is provided at both ends of the upper conductive layer 12. In addition, optically transparent lower substrate 13 is provided with optically transparent lower conductive layer 14 formed on a upper surface thereof in the like manner as the upper conductive layer 12. A pair of lower electrodes 18 is provided on a upper surface of the lower conductive layer 14 at both ends thereof in the like manner as the upper electrodes 17, but in a direction orthogonal to the upper electrodes 17. Also formed are plurality of dot spacers 15 at regular intervals in order to maintain a predetermined space from the upper conductive layer 12. Spacer 16, which develops adhesion by heat curing, is composed of bisphenol A type epoxy resin having 3.2×107 Pa in modulus of elasticity. The spacer 16 is formed into a frame-like shape on any of the lower surface of upper substrate 11 and the upper surface of lower substrate 13 along the periphery thereof. The spacer 16 thus bonds together the upper substrate 11 and the lower substrate 13 along their periphery in a manner that the upper conductive layer 12 and the lower conductive layer 14 confront with respect to each other with a predetermined space between them. The optically transparent touch panel is constructed as described above.
In the structure described above, the upper electrodes 17 and the lower electrodes 18 are connected to a detector circuit of an electronic device. When the user depresses any point in an effective operational area of the touch panel with a finger, pen and the like, the upper substrate 11 deforms around the depressed point. This causes the upper conductive layer 12 to come into contact with the lower conductive layer 14. The depressed point is determined by detecting a ratio of resistances between the upper electrodes 17 as well as that of the lower electrodes 18. In this instance, the effective operational area defines an upper surface area of the upper substrate 11 other than a portion bonded by the spacer 16, and it is the area available for depressing operation and detection of the depressed point.
In the above-discussed optically transparent touch panel of the prior art, however, the upper conductive layer 12 receives a large bending stress around an edge of the spacer 16 when a depressing force is applied in the vicinity of the spacer 16, and this stress becomes greater the closer the depressed point is to the spacer 16. Since this stress tends to accelerate fatigue of the upper conductive layer 12, an area near the spacer 16 cannot be used as the operational area of the optically transparent touch panel. As a result, the effective operational area is restricted by this portion of unusable area. It is therefore inevitable to increase overall dimensions by an amount as great as this unusable area to provide a given size of effective operational area, which makes the whole touch panel bulky.